DE 36 28 544 A1 discloses a magnetic holder for providing attachment to a magnetisable surface, in which the contact assembly is made up of members which are fed from a magnet. The members, flat pole shoe plates, are displaceable in parallel to one another, so that the contact assembly can be closely adapted to any form of surface. After the adaptation of the assembly of members to the contact surface, the individual plate members are locked one with another and fixed in place. The assembly of members is held on the contact surface by magnetic force. It is, however, disadvantageous that with such a holding device the assembly of members must always be newly adapted upon a change of the contact surface, that the magnetic retaining force is relatively small and, in particular, that the variable form of the assembly of members, along with the overall construction of the magnetic holder, affects the measured values of particular kinds of contact sensors, in particular vibration sensors.
DE 29 40 212 C2 describes a magnetic mounting device with at least two soft ferromagnetic pole shoes which are fed from a plurality of magnets. The pole shoes are fed both from a main magnet arranged between them and from surrounding secondary magnets which are so oriented that they offer in each case poles of the same polarity towards the respective pole shoes. The poles of the secondary magnets facing away are bridged by a magnetic yoke. This magnetic holder is characterized by a large magnetic holding force, or by a high magnetic energy density at the holding surface, but in substance this is so only for flat plane surfaces. The above-mentioned magnetic mounting device is not suitable for curved surfaces.
Attachment or holding on a magnetisable surface can also be achieved by a horseshoe magnet or a pot-shaped magnet which, for increasing the holding force and for overcoming unevennesses, can have movable pole shoes at the sides. Such magnetic holders are primarily used as door holders or cupboard door catches. They are, however, of only limited suitability for contacting curved surfaces. The pole shoes at the sides, with the magnet between them, form a bridge when contacted to a curved surface, the self-vibration of which bridge has a negative effect in particular in the case of vibration sensors. The self-vibrations are particularly disruptive when complex vibration data from bearings or shafts is to be evaluated.